Arthur and his knights stumble on a shrine in the woods that Merlin identifies as having belonged to the old religion. The wizard warns his cohorts away, but not before Elyan, succumbing to thirst, takes a drink from the shrine's well. He is immediately haunted by the image of a child, and the visions continue once the group returns to Camelot.
Meanwhile, Arthur and other members of his inner circle are struggling in the aftermath of Gwen's banishment. Arthur takes some of his angst out on Merlin in a training session, but it is Elyan's aggressive attack that leads the knights to realize something is wrong with their compatriot. Meanwhile, the spirit continues to plague the poor knight, claiming the only way it can be given rest is through the killing of the king. Fortunately, Arthur is able to ward off the attack of his possessed friend.
Elyan is captured and thrown in the dungeons, and Agravaine wants the attacker's head, but Merlin believes he can free Elyan from his curse. Unfortunately, his plan doesn't work so well, as Elyan knocks the boy wizard unconscious and escapes to attack the king again. But Arthur realizes what he must do and, in an act that showcases his nobility, sneaks out to the shrine to apologize to the boy and his druidic people for the damage that he, Arthur, caused.
Merlin has pushed the limits of his strength to save Arthur, but even he may not be powerful enough to rescue Camelot€™s king.
In the battle foretold by the prophecies, all give their best, and Merlin wins the day.
Morgana may have one last trick up her sleeve to win the war.